U.S. patent application number 15/012599 was filed with the patent office on 2016-08-04 for method and system for imaging of a photomask through a pellicle.
The applicant listed for this patent is KLA-Tencor Corporation. Invention is credited to Gildardo Delgado, Garry A. Rose, William G. Schultz.
Application Number | 20160225582 15/012599 |
Document ID | / |
Family ID | 56553323 |
Filed Date | 2016-08-04 |
United States Patent
Application |
20160225582 |
Kind Code |
A1 |
Schultz; William G. ; et
al. |
August 4, 2016 |
Method and System for Imaging of a Photomask Through a Pellicle
Abstract
A system for imaging a sample through a protective pellicle is
disclosed. The system includes an electron beam source configured
to generate an electron beam and a sample stage configured to
secure a sample and a pellicle, wherein the pellicle is disposed
above the sample. The system also includes an electron-optical
column including a set of electron-optical elements to direct at
least a portion of the electron beam through the pellicle and onto
a portion of the sample. In addition, the system includes a
detector assembly positioned above the pellicle and configured to
detect electrons emanating from the surface of the sample.
Inventors: |
Schultz; William G.; (San
Jose, CA) ; Delgado; Gildardo; (Livermore, CA)
; Rose; Garry A.; (Livermore, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KLA-Tencor Corporation |
Milpitas |
CA |
US |
|
|
Family ID: |
56553323 |
Appl. No.: |
15/012599 |
Filed: |
February 1, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62111413 |
Feb 3, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 2237/2002 20130101;
H01J 2237/2445 20130101; H01J 37/28 20130101; H01J 2237/2448
20130101; H01J 2237/2449 20130101; H01J 2237/24475 20130101; H01J
2237/2817 20130101; H01J 37/244 20130101; G03F 1/62 20130101; G03F
1/86 20130101 |
International
Class: |
H01J 37/28 20060101
H01J037/28; H01J 37/147 20060101 H01J037/147; H01J 37/26 20060101
H01J037/26 |
Claims
1. A scanning electron microscopy apparatus comprising: an electron
beam source configured to generate an electron beam; a sample stage
configured to secure a sample and a pellicle, wherein the pellicle
is disposed above the sample; an electron-optical column including
a set of electron-optical elements to direct at least a portion of
the electron beam through the pellicle and onto a portion of the
sample; and a detector assembly positioned above the pellicle and
configured to detect electrons emanating from the surface of the
sample.
2. The apparatus of claim 1, wherein the electron beam source
comprises: one or more electron guns.
3. The apparatus of claim 1, wherein the sample comprises: a
photomask.
4. The apparatus of claim 3, wherein the sample comprises: at least
one of an extreme ultraviolet photomask or an X-ray photomask.
5. The apparatus of claim 1, wherein at least one of the pellicle
or sample is conductive.
6. The apparatus of claim 1, wherein the detector assembly
comprises: one or more backscattered electron detectors.
7. The apparatus of claim 6, wherein the detector assembly
comprises: an array of backscattered electron detectors.
8. The apparatus of claim 1, wherein the detector assembly
comprises: one or more secondary electron detectors.
9. The apparatus of claim 8, wherein the detector assembly
comprises: an Everhart-Thornley secondary electron detector.
10. The apparatus of claim 8, wherein the detector assembly
comprises: a secondary electron detector disposed with the
electron-optical column.
11. The apparatus of claim 8, wherein the detector assembly
comprises: a multi-channel electron multiplier detector.
12. The apparatus of claim 1, further comprising: bias control
circuitry to control the potential on at least one of the pellicle
or sample.
13. The apparatus of claim 12, wherein the bias control circuitry
establishes a negative bias on the sample relative to the
pellicle.
14. The apparatus of claim 12, wherein the bias control circuitry
is integrated with the sample stage to establish one or more
electrical connections between the sample stage and at least one of
the pellicle or the sample in order to ground at least one of the
pellicle or the sample.
15. The apparatus of claim 1, further comprising: a controller
communicatively coupled to the detector assembly and configured to
form one or more images of the surface of the sample based on one
or more signals from the detector assembly.
16. A scanning electron microscopy apparatus comprising: an
electron beam source configured to generate an electron beam; a
sample stage configured to secure a sample and a pellicle, wherein
the pellicle is disposed above the sample, wherein a selected gas
is contained within the volume between the pellicle and the
photomask at a selected pressure; and an electron-optical column
including a set of electron-optical elements to direct at least a
portion of the electron beam through the pellicle and onto a
portion of the sample, wherein the selected gas amplifies electrons
emanating from the surface of the sample.
17. The apparatus of claim 16, wherein the selected gas comprises:
at least one of H.sub.2O, O.sub.2, H.sub.2, O.sub.3 or N.sub.2.
18. The apparatus of claim 16, wherein the selected pressure is
between 0.1 to 10 Torr.
19. The apparatus of claim 16, further comprising: bias control
circuitry to control the potential on at least one of the pellicle
or sample.
20. The apparatus of claim 19, wherein the bias control circuitry
establishes a positive bias on the pellicle relative to the
sample.
21. The apparatus of claim 19, wherein the bias control circuitry
is integrated with the sample stage to establish one or more
electrical connections between the sample stage and at least one of
the pellicle or the sample in order to ground at least one of the
pellicle or the sample.
22. The apparatus of claim 16, further comprising: a controller
electrically coupled to at least one of the pellicle or the sample
detector assembly and configured to receive a current output from
at least one of the pellicle or the sample representative of the
electrons absorbed by at least one of the pellicle or the
sample.
23. The apparatus of claim 22, further comprising: one or more
light detectors disposed above the pellicle and configured to
detect photons emitted from the gas contained within the volume
between the pellicle and the photomask.
24. The apparatus of claim 23, wherein the controller is configured
to receive one or more signals from the one or more light detectors
indicative of the detected photons from the gas contained within
the volume between the pellicle and the photomask.
25. The apparatus of claim 24, wherein the controller is configured
to image one or more portions of the sample based on at least one
of the received current output from at least one of the pellicle or
the sample or the one or more received signals from the one or more
light detectors.
26. A method for imaging a sample through a pellicle comprising:
generating an electron beam; directing the electron beam through a
pellicle onto a surface of a sample; and detecting at least one of
backscattered electrons scattered from the surface of the sample,
secondary electrons emitted from the surface of the sample, or
photons emitted by electron-gas interactions within a pressurized
gas between the pellicle and the sample.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims benefit under 35 U.S.C.
.sctn.119(e) and constitutes a regular (non-provisional) patent
application of U.S. Provisional Application Ser. No. 62/111,413,
filed Feb. 3, 2015, entitled POSSIBLE MEANS OF SEM IMAGING OF
PHOTOMASKS THROUGH A PELLICLE, naming William George Schultz,
Gildardo Rio Delgado and Garry Allen Rose as inventors, which is
incorporated herein by reference in the entirety.
TECHNICAL FIELD
[0002] The present invention generally relates to scanning electron
microscopy, and, in particular, a scanning electron microscopy
system for imaging a photomask through a pellicle.
BACKGROUND
[0003] Fabricating semiconductor devices such as logic and memory
devices typically includes processing a substrate such as a
semiconductor wafer using a large number of semiconductor
fabrication processes to form various features and multiple levels
of the semiconductor devices. As semiconductor device size become
smaller and smaller, it becomes critical to develop enhanced
photomask inspection and review devices and procedures.
[0004] Actinic and non-actinic optical microscopy and standard
E-beam inspection systems have been used to inspect photomasks.
Conventional secondary electron detectors include, but are not
limited to, an Everhart-Thornley detector, a multichannel plate, a
PIN detector, an avalanche diode, or APD. These detectors typically
allow for the imaging of low energy secondary electrons from a
metallic or hybrid semiconductor/metallic surface. However, the
implementation of a protective and conductive pellicle film,
positioned above the given photomask, has severely limited the
ability of these conventional imaging approaches to image the
photomask. Further, photomasks used in extreme ultraviolet (EUV)
lithography require detection of defect particles as small as 10 nm
in diameter, which further limits the usefulness of conventional
electron imaging approaches in inspection of EUV-based
photomasks.
[0005] As such, it would be advantageous to provide a system and
method that provides improved electron imaging of photomasks
through a protective pellicle so as to remedy the shortcomings of
the conventional approaches identified above.
SUMMARY
[0006] A scanning electron microscopy (SEM) apparatus is disclosed,
in accordance with one or more embodiments of the present
disclosure. In one illustrative embodiment, the SEM apparatus
includes an electron beam source configured to generate an electron
beam. In another illustrative embodiment, the SEM apparatus
includes a sample stage configured to secure a sample and a
pellicle, wherein the pellicle is disposed above the sample. In
another illustrative embodiment, the SEM apparatus includes an
electron-optical column including a set of electron-optical
elements to direct at least a portion of the electron beam through
the pellicle and onto a portion of the sample. In another
illustrative embodiment, the SEM apparatus includes a detector
assembly positioned above the pellicle and configured to detect
electrons emanating from the surface of the sample.
[0007] A scanning electron microscopy (SEM) apparatus is disclosed,
in accordance with one or more embodiments of the present
disclosure. In one illustrative embodiment, the SEM apparatus
includes an electron beam source configured to generate an electron
beam. In another illustrative embodiment, the SEM apparatus
includes a sample stage configured to secure a sample and a
pellicle, wherein the pellicle is disposed above the sample and a
selected gas is contained within the volume between the pellicle
and the photomask at a selected pressure. In another illustrative
embodiment, the SEM apparatus includes an electron-optical column
including a set of electron-optical elements to direct at least a
portion of the electron beam through the pellicle and onto a
portion of the sample. In another illustrative embodiment, the
selected gas amplifies electrons emanating from the surface of the
sample.
[0008] A method for imaging a sample through a pellicle is
disclosed, in accordance with one or more embodiments of the
present disclosure. In one illustrative embodiment, the method
includes generating an electron beam. In another illustrative
embodiment, the method includes directing the electron beam through
a pellicle onto a surface of a sample. In another illustrative
embodiment, the method includes detecting at least one of
backscattered electrons scattered from the surface of the sample,
secondary electrons emitted from the surface of the sample, or
photons emitted by electron-gas interactions within a pressurized
gas between the pellicle and the sample.
[0009] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not necessarily restrictive of the
invention as claimed. The accompanying drawings, which are
incorporated in and constitute a part of the specification,
illustrate embodiments of the invention and together with the
general description, serve to explain the principles of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The numerous advantages of the disclosure may be better
understood by those skilled in the art by reference to the
accompanying figures in which:
[0011] FIG. 1A is a high level schematic illustration of a system
for imaging a photomask through a protective pellicle via the
collection of backscattered electrons, in accordance with one
embodiment of the present disclosure.
[0012] FIG. 1B is a top view of a backscattered electron detector,
in accordance with one embodiment of the present disclosure.
[0013] FIG. 1C is a top view of a backscattered electron quad
detector, in accordance with one embodiment of the present
disclosure.
[0014] FIG. 1D is a high level schematic illustration of a system
for imaging a photomask through a protective pellicle via the
collection of backscattered electrons, in accordance with one
embodiment of the present invention.
[0015] FIG. 1E is a high level schematic illustration of a system
for imaging a photomask through a protective pellicle via the
collection of secondary electrons with an Everhart-Thornley
secondary electron detector, in accordance with one embodiment of
the present disclosure.
[0016] FIG. 1F is a high level schematic illustration of a system
for imaging a photomask through a protective pellicle via the
collection of secondary electrons with an in-column secondary
electron detector, in accordance with one embodiment of the present
disclosure.
[0017] FIG. 1G is a high level schematic illustration of a system
for imaging a photomask through a protective pellicle via the
measurement of current in the pellicle or photomask caused by the
absorption of backscattered electrons by the pellicle or photomask,
in accordance with one embodiment of the present disclosure.
[0018] FIG. 1H is a high level schematic illustration of a system
for imaging a photomask through a protective pellicle via the
measurement of current in the pellicle or photomask caused by the
absorption of secondary electrons by the pellicle or photomask, in
accordance with one embodiment of the present disclosure.
[0019] FIG. 1I is a high level schematic illustration of a system
for imaging a photomask through a protective pellicle via the
measurement of the gas cascade amplified secondary electrons
absorbed by the pellicle or the measurement of the gas cascade
secondary electrons leaving the photomask, in accordance with one
embodiment of the present disclosure.
[0020] FIG. 1J is a high level schematic illustration of a system
for imaging a photomask through a protective pellicle via the
measurement of the gas cascade amplified secondary electrons
absorbed by the pellicle, the measurement of the gas cascade
secondary electrons leaving the photomask or the collection of
photons resulting from electron-gas interactions in a pressurized
gas medium, in accordance with one embodiment of the present
disclosure.
[0021] FIG. 2 is a process flow diagram illustrating a method for
imaging one or more portions of a photomask through a protective
pellicle, in accordance with one embodiment of the present
disclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Reference will now be made in detail to the subject matter
disclosed, which is illustrated in the accompanying drawings.
Referring generally to FIGS. 1A through 1K, a system and method for
imaging a photomask is described in accordance with the present
disclosure. Embodiments of the present disclosure are directed to a
scanning electron microscopy (SEM) system capable of imaging a
photomask through a pellicle positioned to protect the underlying
photomask. Embodiments of the present disclosure are directed to
the collection of backscattered electrons scattered from the
surface of the photomask and transmitted through the pellicle.
Additional embodiments of the present disclosure are directed to
the collection of secondary electrons emitted by the photomask and
transmitted through the pellicle. Additional embodiments of the
present disclosure are directed to the collection of secondary
electrons resulting from the amplification of initial "weak"
secondary electrons by a pressurized gas medium and/or the
collection of photons resulting from gas-electron interactions
within the pressurized gas medium.
[0023] The system 100 may be used to inspect and/or review any
sample known in the art of scanning electron microscopy. For
example, the sample may include any photomask known in the art,
such as, but not limited to, EUV multilayer (ML) photomask or an
X-ray photomask. For instance, an EUV ML photomask may include, but
is not limited to, a Mo/Si multilayer reflective mask.
[0024] It is noted that while the system and method of the present
disclosure are discussed in the context of photomask
inspection/review this should not be interpreted as a limitation on
the scope of the present disclosure. It is recognized herein that
the embodiments of the present disclosure may be extended to image
any type of sample through any type of protective element, such as,
but not limited to, a pellicle. For example, embodiments of the
present disclosure may be adapted to perform inspection and/or
defect review on a wafer (e.g., semiconductor wafer) that is
protected by a pellicle, membrane or film.
[0025] Embodiments of the present disclosure may image one or more
portions of a photomask through an overlying pellicle using
information gathered from the collected backscattered electrons,
secondary electrons and/or photons. Based on the imaging of the
surface of the photomask, embodiments of the present disclosure may
identify defects present on the surface of the photomask. In the
case of EUV ML masks, such defects include, but are not limited to,
phase defects, absorber pattern defects and haze formation. ML
phase defects are primarily caused by the inclusion of defects on
the substrate during ML deposition of the substrate. Even a few
nanometer height differences on the ML mask may cause a printable
phase defect because of the short wavelength of 13.5 nm utilized in
EUV lithography. Haze formation is commonly caused by ammonium
salts (e.g., ammonium sulfate, ammonium nitrates, ammonium
phosphates, ammonium oxalate), organics and siloxanes film growth.
Haze formation occurs at preferential sites on photomask and is
often material and structure dependent.
[0026] Embodiments of the present disclosure utilize a conductive
pellicle and/or a conductive photomask as electron optical elements
of system 100, which aids in defect review by applying electrical
charges to the pellicle and/or photomask. By establishing a
negative charge on the pellicle it is possible to establish a
retarding field, which decelerates primary beam electrons before
impinging on the photomask surface. In addition, the pellicle may
be positively charged to accelerate secondary electrons emitted by
the surface of the photomask. Further, the charge on the photomask
may be controlled in order to control the landing energy of
electrons incident on the photomask. These features are discussed
in greater detail further herein.
[0027] FIGS. 1A-1D illustrates system 100 arranged for imaging a
photomask protected by a pellicle via the collection of
backscattered electrons, in accordance with one embodiment of the
present disclosure.
[0028] In one embodiment, the system 100 includes an electron beam
source 102 for generating one or more electron beams 104. The
electron beam source 102 may include any electron source known in
the art. For example, the electron beam source 102 may include, but
is not limited to, one or more electron guns. For instance, the
electron beam source 102 may include a single electron gun for
generating a single electron beam 104. In another instance, the
electron beam source 102 may include multiple electron guns for
generating multiple electron beams 104. For example, the energy of
the electron beam formed by the electron beam source 102 may be
between 1 and 20 kV. It is noted that the energy of the beam 104 is
not limited to 1-20 kV, which is provided merely for illustrative
purposes. It is recognized herein that the energy of the primary
beam 104 may reach 200 kV.
[0029] In another embodiment, the system 100 includes a sample
stage 111. The sample stage 111 secures the photomask 110 and the
pellicle 108. It is noted that the pellicle 108 is disposed above
the photomask 111. In one embodiment, the pellicle 108 is secured
above the photomask 110 with frame 113. As discussed further
herein, the pellicle 108 and photomask 110 may be electrically
isolated from one another (and the rest of the system), allowing
for biasing of the pellicle 108 and photomask 110 relative to one
another. It is noted that the terms "above" and "below," as used
throughout the present disclosure, are used for purposes of
simplicity only and are not meant to be interpreted as a limitation
on the present disclosure.
[0030] In another embodiment, the sample stage 111 is an actuatable
stage. For example, the sample stage 111 may include, but is not
limited to, one or more translational stages suitable for
selectably translating the photomask 110 along one or more linear
directions (e.g., x-direction, y-direction and/or z-direction). By
way of another example, the sample stage 111 may include, but is
not limited to, one or more rotational stages suitable for
selectably rotating the photomask 110 along a rotational direction.
By way of another example, the sample stage 108 may include, but is
not limited to, a rotational stage and a translational stage
suitable for selectably translating the sample along a linear
direction and/or rotating the photomask 110 along a rotational
direction.
[0031] In another embodiment, the system 100 includes an
electron-optical column 106. The electron-optical column 106 may
include a set of electron-optical elements. The set of
electron-optical elements may direct at least a portion of the
electron beam 104 through the pellicle 108 and onto a selected
portion of the photomask 110. The set of electron-optical elements
of the electron-optical column 106 may include any electron-optical
elements known in the art suitable for focusing and/or directing
the electron beam 104 through the pellicle 108 and onto a selected
portion of the photomask 110. In one embodiment, the set of
electron-optical elements includes one or more electron-optical
lenses. For example, the electron-optical lenses may include, but
are not limited to, one or more condenser lenses 114 for collecting
electrons from the electron beam source 102. By way of another
example, the electron-optical lenses may include, but are not
limited to, one or more objective lenses 115 for focusing the
electron beam 104 onto a selected region of the photomask 110.
[0032] For purposes of simplicity a single electron-optical column
106 is depicted in FIG. 1A. It is noted herein that this
configuration should not be interpreted as a limitation the present
disclosure. For example, the system 100 may include multiple
electron-optical columns 106.
[0033] In another embodiment, the set of electron-optical elements
of the electron-optical column 106 includes one or more electron
beam scanning elements 116. For example, the one or more electron
beam scanning elements 116 may include, but are not limited to, one
or more electromagnetic scanning coils or electrostatic deflectors
suitable for controlling a position of the beam 104 relative to the
surface of the photomask 110. In this regard, the one or more
scanning elements 116 may be utilized to scan the electron beam 104
across the photomask 110 in a selected pattern.
[0034] In another embodiment, the system 100 includes a
backscattered electron detector assembly 112. The backscattered
electron detector assembly 112 may include any detector technology
known in the art capable of detecting backscattered electrons. For
example, the backscattered electron detector assembly 112 may be
positioned below the electron-optical column 106 and above the
pellicle 108. In one embodiment, as shown in FIG. 1B, the detector
assembly 112 may include a single annular backscattered electron
detector. In another embodiment, as shown in FIG. 1C, the detector
assembly 112 may include a multi-element annular backscattered
electron detector. For example, as shown in FIG. 1C, the detector
assembly 112 includes, but is not limited to, a backscattered
electron quad array including elements 113a-113d. It is noted that
the use of an array a backscattered electron detectors (e.g., quad
detector in FIG. 1C) allows for the determination of topography
and/or composition of the photomask 110.
[0035] It is noted that that the backscattered electron detector
assembly 112 may include any type of backscattered electron
detector known in the art. In one embodiment, backscattered
electrons may be collected and imaged using a Everhart-Thornley
detector (or other type of scintillator-based detector). In another
embodiment, backscattered electrons may be collected and imaged
using a micro-channel plate (MCP). In another embodiment,
backscattered electrons may be collected and imaged using PIN or
p-n junction detector, such as a diode or a diode array. In another
embodiment, backscattered electrons may be collected and imaged
using one or more avalanche photo diodes (APDs).
[0036] It is noted herein that the system 100 may operate in any
scanning mode known in the art. For example, the system 100 may
operate in a swathing mode when scanning an electron beam 104
across the surface of the photomask 110. In this regard, the system
100 may scan an electron beam 104 across the photomask 110, while
the sample is moving, with the direction of scanning being
nominally perpendicular to the direction of the sample motion. By
way of another example, the system 100 may operate in a
step-and-scan mode when scanning an electron beam 104 across the
surface of the photomask 110. In this regard, the system 100 may
scan an electron beam 104 across the photomask 110, which is
nominally stationary when the beam 104 is being scanned.
[0037] The system 100 may extract surface defect data from a
photomask using a high beam energy electron beam 104 from the
electron beam source 102 that penetrates the pellicle film 108 and
continues to the surface of the photomask 110. These electrons will
elastically collide with nuclei of the photomask material and
backscatter from the surface of the photomask 110. The
backscattered electron (BSE) signal emerges from within the bulk of
the surface of the photomask 110 with a given distribution (e.g., a
cosine distribution). It is noted that BSE imaging may display a
hard resolution limit related to the beam energy and the target
material. However, the high energy backscattered electron signal,
emerging from deep within the photomask surface, may be
sufficiently energetic to traverse the pellicle 108 to a detector
assembly 112 located elsewhere within the system 100.
[0038] In another embodiment, the various components of system 100
are disposed within a vacuum chamber (not shown). In order to avoid
damage or contamination of the photomask 110 all vacuum system
components, electrical and mechanical feedthroughs, connectors and
cable/wire assemblies for the system 100 are constructed from
approved materials. Materials that may prove problematic in the
vacuum system include, but are not limited to, Hg, Tl, Se, Te, Cd,
Au, Ag, In, Zn, Sn, Pb, S, Silicon oils and greases and Silicon
based adhesives and epoxies. In addition, commonly used polymer
(plastics) and elastomer materials such as, but not limited to,
neoprene, adaprene, urethane, polyurethane, polyester, silicone,
polypropylene, polystyrene, polyethylene, nylon, polycarbonates,
polyolefins and Molybdenum disulfide (MoS2) should be avoided or at
least well regulated. Any problematic materials should be enclosed
so they do not outgas, eject particles or adversely interact with
the electron beam.
[0039] FIGS. 1E-1F illustrate system 100 configured for imaging a
photomask protected by a pellicle via the collection of secondary
electrons, in accordance with an additional embodiment of the
present disclosure. It is noted herein that the various examples
and embodiments described previously herein with respect to FIGS.
1A-1D should be interpreted to extend to the embodiments of FIGS.
1E-1F unless otherwise noted. In one embodiment, the system 100
includes a secondary electron detector assembly 122. The secondary
electron detector assembly 122 may include any detector technology
known in the art capable of detecting secondary electrons. For
example, as shown in FIG. 1E, the secondary electron detector
assembly 122 may include, but is not limited to, an
Everhart-Thornley detector. For instance, the detector assembly 122
may include an electron collector 126 (e.g., secondary electron
collector), which may be biased to collect secondary electrons 125
emitted by the surface of the photomask 110. Further, the detector
assembly 122 includes a detector element 127 (e.g., scintillating
element and PMT detector) for detecting electrons 125 from the
photomask surface. By way of another example, as shown in FIG. 1F,
the secondary electron detector assembly 122 may include, but is
not limited to, an in-column detector. For instance, the detector
assembly 122 may include a secondary electron detector disposed
within the electron-optical column 106. By way of another example,
the secondary electron detector may include, but is not limited to,
a multi-channel electron multiplier. By way of another example, the
secondary electron detector may include, but is not limited to, one
or more PIN diodes or one or more avalanche photodiodes (APDs).
[0040] In another embodiment, a retarding voltage is established
between the pellicle 108 and the photomask 110. In one embodiment,
the retarding voltage is established by negatively biasing the
photomask 110 relative to the pellicle 108. For example, the
pellicle 108 may be grounded, with the photomask 110 held at a
negative potential. For example, as shown in FIGS. 1E and 1F, the
system 100 includes bias control circuitry 118. The bias control
circuitry 118 may connect the pellicle 108 to ground, while
establishing a negative potential on the photomask 110 (e.g., via a
voltage source). In one embodiment, the bias control circuitry 118
is integrated with the sample stage 111 to establish one or more
electrical connections between the sample stage 111 and the
pellicle 108 and/or photomask 110 in order to ground the pellicle
108, while providing a negative potential to the photomask 110
(e.g., via a voltage source).
[0041] In one embodiment, the retarding voltage serves to
decelerate electrons in the beam 104 when impacting the surface of
the photomask 110. The deceleration of electrons incident on the
photomask surface increases the sensitivity of the system 100 to
smaller surface detail. In turn, when secondary electrons 125 are
emitted by the photomask 110 they are accelerated back to the
pellicle 108 with sufficient voltage to penetrate and traverse the
pellicle 108. Next, once the secondary electrons emerge from the
pellicle 108, they are collected by the detector assembly 112.
[0042] FIG. 1G illustrates system 100 configured for imaging the
photomask 110 via the collection of backscattered electrons and/or
the current signal resulting from the absorption of electrons by
the pellicle 108 or photomask 110, in accordance with one
embodiment of the present disclosure.
[0043] In one embodiment, the current induced in the conductive
pellicle 108 by the backscattered electrons 123 absorbed by the
pellicle 108 is measured. For example, the system 100 may include
one or more current amplifiers 131 coupled to the pellicle 108 for
amplifying the current from the pellicle 108. In this regard, as
the primary beam 104 is scanned across the photomask 110 (and
through the pellicle 108) the controller 132 may register an
amplified output current from the pellicle 108 via amplifier 131.
In turn, the controller 132 may image one or more portions of
surface of the photomask 110 with the measured current, which is
induced by the absorption of backscattered electrons 123 by the
pellicle 108.
[0044] In another embodiment, the current induced in the conductive
photomask 110 by the electrons absorbed by photomask 110 is
measured. For example, the system 100 may include one or more
current amplifiers 133 coupled to the photomask 110 for amplifying
the current from the photomask 110. In this regard, as the primary
beam 104 is scanned across the photomask 110 the controller 132 may
register an amplified output current from the photomask 110 via
amplifier 133. In turn, the controller 132 may image one or more
portions of surface of the photomask 110 with the measured current,
which is induced by the absorption of electrons (i.e., electrons
not scattered by photomask) by the photomask 110.
[0045] In one embodiment, system 100 includes a controller 132. The
controller 132 may be communicatively coupled to the output of the
backscattered electron detector 112 and/or current amplifier 131
and/or current amplifier 133.
[0046] In another embodiment, the controller 132 may form an image
of one or more portions of the photomask 110 based on a combination
of the measured current from current amplifiers 131 and/or 133 and
the measured backscattered electron signal from the one or more
backscattered electron detectors 112. In this regard, the
controller 132 may combine the signals in any manner known in the
art. For instance, after calibrating the current and/or electron
signals, the controller 132 may add or subtract the signals to form
a composite signal. In this regard, the scattered backscattered
signal and the absorbed electron signal can be combined or
subtracted.
[0047] FIG. 1H illustrates system 100 configured for imaging the
photomask 110 via the collection of secondary electrons and/or the
current signal resulting from the absorption of electrons by the
pellicle 108 or electrons leaving photomask 110, in accordance with
one embodiment of the present disclosure, in accordance with one
embodiment of the present disclosure.
[0048] In one embodiment, the current induced in the conductive
pellicle 108 by the secondary electrons 125 absorbed by the
pellicle 108 is measured. For example, the one or more current
amplifiers 131 coupled to the pellicle 108 may amplify the current
from the pellicle 108. As the primary beam 104 is scanned across
the photomask 110 the controller 132 may register an amplified
output current from the pellicle 108 via amplifier 131. In turn,
the controller 132 may image one or more portions of surface of the
photomask 110 with the measured current, which is induced by the
absorption of secondary electrons 125 by the pellicle 108.
[0049] In another embodiment, the current induced in the conductive
photomask 110 due to the secondary electrons leaving the photomask
110 is measured. For example, the one or more current amplifiers
133 coupled to the photomask 110 may amplify the current from the
photomask 110. As the primary beam 104 is scanned across the
photomask 110 the controller 132 may register an amplified output
current from the photomask 110 via amplifier 133. In turn, the
controller 132 may image one or more portions of surface of the
photomask 110 with the measured current, which is induced as a
result of secondary electrons 125 leaving the photomask 110.
[0050] In one embodiment, system 100 includes a controller 132. The
controller 132 may be communicatively coupled to the output of the
secondary electron detector 122 and/or current amplifier 131 and/or
current amplifier 133.
[0051] In another embodiment, the controller 132 may form an image
of one or more portions of the photomask 110 based on a combination
of the measured current from current amplifiers 131 and/or 133 and
the measured secondary electron signal from the one or more
secondary electron detectors 122. In this regard, the controller
132 may combine the signals in any manner known in the art. For
instance, after calibrating the current and/or electron signals,
the controller 132 may add or subtract the signals to form a
composite signal.
[0052] In one embodiment, the controller 132 includes one or more
processors (not shown) configured to execute program instructions
suitable for causing the one or more processors to execute one or
more steps described in the present disclosure. In one embodiment,
the one or more processors of the controller 102 may be in
communication with a carrier medium (e.g., non-transitory storage
medium (i.e., memory medium)) containing the program instructions
configured to cause the one or more processors of the controller
132 to carry out various steps described through the present
disclosure. It should be recognized that the various processing
steps described throughout the present disclosure may be carried
out by a single computing system or, alternatively, a multiple
computing system. The controller 132 may include, but is not
limited to, a personal computer system, mainframe computer system,
workstation, image computer, parallel processor, or any other
device known in the art. In general, the term "computer system" may
be broadly defined to encompass any device having one or more
processors, which execute instructions from a memory medium.
Moreover, different subsystems of the system 100 may include a
computer system or logic elements suitable for carrying out at
least a portion of the steps described above. Therefore, the above
description should not be interpreted as a limitation on the
present disclosure but merely an illustration.
[0053] FIGS. 1I-1J illustrate system 100 configured for imaging a
photomask protected by a pellicle via the collection of an
amplified electron signal resulting from the gas amplification of
initial secondary electrons, in accordance with an additional
embodiment of the present disclosure. It is noted that the
secondary electrons initially emitted by the photomask 110 may have
low voltage (e.g., 3-5 eV) and, thus, have insufficient energy for
absorption by the pellicle 108.
[0054] In one embodiment, a pressurized gas medium 136 is
maintained with the volume between the pellicle 108 and photomask
110. The pressurized gas medium 136 serves to amplify weak initial
secondary electrons emitted by the surface of the photomask 110 via
an electron cascade process. In this regard, after the primary
electron beam 104 (e.g., energy of 5 to 200 kV) impinges on the
surface of the photomask 110, the photomask 110 emits initial
secondary electrons. In turn, these initial secondary electrons
interact with the pressurized gas medium (i.e., electrons collide
with gas molecules) and emit additional electrons (and photons).
This process is repeated in a cascading process, resulting in a
secondary electron signal significantly larger than the initial
secondary electron signal. An amplification factor of 10,000 may be
achieved. The pressurized gas 136 may include any gas suitable for
sustaining ionization and the cascading of electrons. In addition,
it is desirable to minimize the amount organic material present in
the gas medium 136. The pressurized gas may include, but is not
limited to, H.sub.2O, O.sub.2, H.sub.2, O.sub.3 or N.sub.2.
Further, the gas 136 may be held at a pressure between 0.1 and 10
Torr. It is further noted that the ionizing medium created by the
electron-gas interactions may serve as a cleaning agent to clean
and/or maintain the cleanliness of the photomask 110.
[0055] In another embodiment, an accelerating voltage is
established between the pellicle 108 and the photomask 110. In one
embodiment, the accelerating voltage is established by positively
biasing the pellicle relative to the photomask 110. For example,
the photomask 110 may be grounded, with the pellicle 108 held at a
positive potential. For example, as shown in FIGS. 1I and 1J, the
bias control circuitry 118 may connect the photomask 110 to ground,
while establishing a positive potential on the pellicle 108 (e.g.,
via a voltage source). The bias control circuitry 118 may be
integrated with the sample stage 111 to establish one or more
electrical connections between the sample stage 111 and the
pellicle 108 and/or photomask 110 in order to ground the photomask
110, while providing a positive potential to the pellicle 108. The
acceleration voltage serves to accelerate the gas amplified cascade
electrons 129 toward the pellicle 108, which causes increased
absorption of the gas amplified cascade electrons 129 by the
pellicle 108.
[0056] It is noted that the pellicle 108 may be biased with any
voltage suited for generating an adequate gas amplified cascade
electron signal at the pellicle 108. For example, the pellicle 108
may be biased with a voltage between +0.1 V and 1000 V.
[0057] In another embodiment, the current induced in the conductive
pellicle 108 by the gas amplified cascade electrons 129 is
measured. For example, the system 100 may include one or more
current amplifiers 131 coupled to the pellicle 108 for amplifying
the current from the pellicle 108. In this regard, as the primary
beam 104 is scanned across the photomask 110 (and through the
pellicle 108) the controller 132 may register an amplified output
current from the pellicle 108 via amplifier 131. In turn, the
controller 132 may image one or more portions of surface of the
photomask 110 with the measured current, which is induced by the
absorption of gas amplified cascade electrons by the pellicle
108.
[0058] In another embodiment, the current induced in the conductive
photomask due to the gas amplified cascade electrons 129 leaving
the photomask 110 is measured. For example, the system 100 may
include one or more current amplifiers 133 coupled to the photomask
110 for amplifying the current from the photomask 110. In this
regard, as the primary beam 104 is scanned across the photomask 110
the controller 132 may register an amplified output current from
the photomask 110 via amplifier 133. In turn, the controller 132
may image one or more portions of surface of the photomask 110 with
the measured current, which is when the gas amplified cascade
electrons leave the photomask 110.
[0059] In another embodiment, as shown in FIG. 1J, the system 100
includes one or more light detectors 134. The one or more light
detectors 134 are situated to collection photons emitted from the
gas medium 136, which result from the electron-gas interaction. For
example, the one or more light detectors 134 may include, but are
not limited to, a photomultiplier tube or an avalanche
photodetector. In another embodiment, the controller 132 is
communicatively coupled to the one or more light detectors 134 and
is configured to receive one or more signals indicative of the
measured photon signal from the one or more light detectors 134. In
turn, the controller 132 may image one or more portions of the
surface of the photomask 110 using the received photon signal from
the one or more light detectors.
[0060] In another embodiment, the controller 132 may form an image
of one or more portions of the photomask 110 based on a combination
of the measured current from current amplifiers 131 and/or 133 and
the measured photon signal from the one or more light detectors
134. In this regard, the controller 132 may combine the signals in
any manner known in the art. For instance, after calibrating the
current and/or light signals, the controller 132 may add or
subtract the signals to form a composite signal.
[0061] The use of a gas medium to enhance SEM detection is
described generally in U.S. Pat. No. 4,992,662 to Danilatos, issued
on Feb. 12, 1991, which is incorporated herein by reference in the
entirety. The use of a gas medium to enhance SEM detection is also
described in U.S. Pat. No. 5,362,964 to Knowles et al., issued on
Nov. 8, 1994, which is incorporated herein by reference in the
entirety.
[0062] The embodiments of the system 100 illustrated in FIGS. 1A-1J
may be further configured as described herein. In addition, the
system 100 may be configured to perform any other step(s) of any of
the method embodiment(s) described herein.
[0063] FIG. 2 is a flow diagram illustrating steps performed in a
method for imaging a sample through a protective pellicle. It is
recognized that steps of the process flow 200 may be carried out
via one or more embodiments of system 100. It should, however, be
recognized by those skilled in the art, that the system 100 should
not be interpreted as a limitation on process 200 as it is
contemplated that a variety of system configurations may carry out
process flow 200.
[0064] In a first step 202, an electron beam is generated. For
example, as shown in FIG. 1A, an electron beam 104 may be generated
using an electron beam source 102.
[0065] In a second step 204, the electron beam is directed through
a pellicle onto a surface of a sample. For example, as shown in
FIGS. 1A-1J, the electron-optical elements of the electron-optical
column 106 direct the beam 104 through the pellicle 108 and onto
the surface of the sample, such as, but not limited to, the
photomask 110.
[0066] In a third step 206, the backscattered electrons, secondary
electrons and/or photons that are transmitted back through the
pellicle 108 are detected. For example, as shown in FIG. 1A,
backscattered electrons 123 may scatter from the surface of the
photomask 110 and traverse the pellicle 108. After the
backscattered electrons 123 are transmitted through the pellicle
108, one or more backscattered electron detectors 112 may collect
the backscattered electrons. Controller 132 may use the collected
backscattered electron signal to image one or more portions of the
surface of the photomask 110.
[0067] By way of another example, as shown in FIG. 1E, secondary
electrons 125 may be emitted from the surface of the photomask 110
and traverse the pellicle 108. After the secondary electrons 125
are transmitted through the pellicle 108, one or more secondary
electron detectors 122 may collect the secondary electrons 125.
Controller 132 may use the collected secondary electron signal to
image one or more portions of the surface of the photomask 110.
[0068] By way of another example, as shown in FIGS. 1I and 1J,
initial "weak" secondary electrons may be emitted from the surface
of the photomask 110 and amplified/multiplied by the pressurized
gas medium 136. The electrons generated by gas amplification may
then impinge the pellicle 108 or be absorbed by the photomask 110
itself. After the electrons 135 generated by gas amplification are
absorbed by the pellicle 108 and/or emitted by the photomask 110,
the current is measured (e.g., via current amplifiers 131 and/133
and controller 132). Controller 132 may use the measured current
associated with electrons absorbed by the pellicle 108 and/or the
electrons leaving the photomask 110 to image one or more portions
of the surface of the photomask 110.
[0069] By way of another example, as shown in FIG. 1J, photons 137
may be emitted from the gas medium 136 as a result of electron-gas
interactions. After the photons 137 generated by be electron-gas
interactions are transmitted through the optically transparent
pellicle 108, one or more light detectors 134 (e.g., one or more
photomultiplier tubes) may collect the photons 137. Controller 132
may use the collected photon signal to image one or more portions
of the surface of the photomask 110.
[0070] It is further noted that the method 200 and system 100 may
use any combination of the various detection modes described herein
to image the sample. For example, in the case of gas amplification,
the controller 132 may form an image of one or more portions of the
photomask 110 based on a combination of the measured current signal
from current amplifiers 131 and/or 133 and the measured photon
signal from the one or more light detectors 134. In this regard,
the controller 132 may combine the signal in any manner known in
the art. For instance, after calibrating the current and/or light
signals, the controller 132 may add or subtract the signals to form
a composite signal. It is further noted that the measured current
signal from amplifier 131 and/or amplifier 133 may also be combined
with the backscattered electron measurements from the backscattered
electron detector 112 (see FIG. 1G) and/or the secondary electron
measurements from the secondary electron detector 122 (see FIG.
1H).
[0071] All of the methods described herein may include storing
results of one or more steps of the method embodiments in a storage
medium. The results may include any of the results described herein
and may be stored in any manner known in the art. The storage
medium may include any storage medium described herein or any other
suitable storage medium known in the art. After the results have
been stored, the results can be accessed in the storage medium and
used by any of the method or system embodiments described herein,
formatted for display to a user, used by another software module,
method, or system, etc. Furthermore, the results may be stored
"permanently," "semi-permanently," temporarily, or for some period
of time. For example, the storage medium may be random access
memory (RAM), and the results may not necessarily persist
indefinitely in the storage medium.
[0072] Those having skill in the art will recognize that the state
of the art has progressed to the point where there is little
distinction left between hardware and software implementations of
aspects of systems; the use of hardware or software is generally
(but not always, in that in certain contexts the choice between
hardware and software can become significant) a design choice
representing cost vs. efficiency tradeoffs. Those having skill in
the art will appreciate that there are various vehicles by which
processes and/or systems and/or other technologies described herein
can be effected (e.g., hardware, software, and/or firmware), and
that the preferred vehicle will vary with the context in which the
processes and/or systems and/or other technologies are deployed.
For example, if an implementer determines that speed and accuracy
are paramount, the implementer may opt for a mainly hardware and/or
firmware vehicle; alternatively, if flexibility is paramount, the
implementer may opt for a mainly software implementation; or, yet
again alternatively, the implementer may opt for some combination
of hardware, software, and/or firmware. Hence, there are several
possible vehicles by which the processes and/or devices and/or
other technologies described herein may be effected, none of which
is inherently superior to the other in that any vehicle to be
utilized is a choice dependent upon the context in which the
vehicle will be deployed and the specific concerns (e.g., speed,
flexibility, or predictability) of the implementer, any of which
may vary. Those skilled in the art will recognize that optical
aspects of implementations will typically employ optically-oriented
hardware, software, and or firmware.
[0073] Those skilled in the art will recognize that it is common
within the art to describe devices and/or processes in the fashion
set forth herein, and thereafter use engineering practices to
integrate such described devices and/or processes into data
processing systems. That is, at least a portion of the devices
and/or processes described herein can be integrated into a data
processing system via a reasonable amount of experimentation. Those
having skill in the art will recognize that a typical data
processing system generally includes one or more of a system unit
housing, a video display device, a memory such as volatile and
non-volatile memory, processors such as microprocessors and digital
signal processors, computational entities such as operating
systems, drivers, graphical user interfaces, and applications
programs, one or more interaction devices, such as a touch pad or
screen, and/or control systems including feedback loops and control
motors (e.g., feedback for sensing position and/or velocity;
control motors for moving and/or adjusting components and/or
quantities). A typical data processing system may be implemented
utilizing any suitable commercially available components, such as
those typically found in data computing/communication and/or
network computing/communication systems.
[0074] It is believed that the present disclosure and many of its
attendant advantages will be understood by the foregoing
description, and it will be apparent that various changes may be
made in the form, construction and arrangement of the components
without departing from the disclosed subject matter or without
sacrificing all of its material advantages. The form described is
merely explanatory, and it is the intention of the following claims
to encompass and include such changes.
* * * * *